1. Field of the Invention
This invention relates to a laundry appliance.
2. Description of the Prior Art
Conventional horizontal axis washing machines involve a final spin cycle to extract as much water as possible from the washed articles to reduce the drying time. However, the requirement of a high spin speed is at odds with quiet operation. At the beginning of a spin the cycle the wash load can be quite severely unbalanced, such that when the machine tries to accelerate noise and stressful vibrations result.
The means that washing machine designers have employed so far to cater for imbalance in the load, is typically to suspend the internal assembly on springs and dampers in order to isolate its vibration. The difficulty is these suspension assemblies never isolate the vibration completely, and as the machine ages they deteriorate. Also, these suspension assemblies require significant internal clearance, and so valuable load capacity is lost when designing a machine to standard outside dimensions. Further, because the internal assembly must still withstand the forces due to the imbalance, considerable extra costs result.
Present machines also try to eliminate the problem at its source, for which there are various solutions. The first possibility is to ensure that the wash load is more evenly distributed prior to spinning. This is effective at reducing the imbalance, but does not usually eliminate the imbalance. At high spin speeds, even small imbalances create large vibrations. Therefore while steps can be taken to reduce the degree of imbalance, it is not possible to eliminate it sufficiently to ignore it there after. So these techniques are usually used in conjunction with the suspended tub systems.
Another approach is to determine the size and nature of the imbalance, and add a balance mass that counteracts the imbalance.
Methods of compensating for imbalance in horizontal axis washing machines have been disclosed in U.S. Pat. No. 5,280,660 (Pellerin et al.), European Patent 856604 (Fagor, S. Coop). These disclosures relate to the use of three axially orientated chambers running the length of the drum, placed evenly around the periphery of the drum. These chambers can be individually filled with water in appropriate amounts to approximately correct the imbalance.
The disadvantage to these systems is that the imbalance may not be centered along the axis of rotation, and since no control is available along the axis of rotation this form of balancing will only ever be partially successful. This may mean that a suspension system is still required to isolate the vibration.
Static Imbalance
When an object of some shape or form is spun about a particular axis, the object mass exhibits static and dynamic imbalance. Static imbalance is where the axis of rotation does not pass through the centre of gravity (CoG) of the object. This means that a force must be applied to the object (acting through the CoG) to keep accelerating the object towards the axis of rotation. This force (F) must come from the surrounding structure and the direction of the force rotates with the object, as illustrated in FIG. 1. There are two pieces of information required to define a static imbalance 3. They are the magnitude of the imbalance 1 (the moment of the CoG about the spin axis, which in SI units has dimensions kg m), and some angle 2 between the direction of the offset of the CoG and some reference direction within the object 4.
When mounted to have a horizontal rotation axis, and allowed to rotate under the influence of gravity, an object with a static imbalance will rotate until its CoG lies vertically under its axis of rotation. This also has the consequence that a horizontal axis machine, running at speeds slower than its resonance on its suspension and at constant power input, will exhibit a slight fluctuation in rotation speed as the CoG goes up one side and down the other. Unfortunately this is not a feasible technique for determining static imbalance at anything other than very slow speeds.
Dynamic Imbalance
Dynamic imbalance is more complex. In FIG. 2 the axis of rotation 5 is not parallel with one of the principle axes 6 of the object. The principal axes of an object are the axes about which the object will naturally spin.
For example, a short length of uniform cylinder 7 set to spin about its axis of extrusion is both statically and dynamically balanced. If two weights are attached to the inside of the cylinder, one 8 at one end and the other 9 at the other end but on the opposite side from the first one the CoG 10 of the object has not been moved and so the object is still statically balanced. However now spinning the cylinder will cause vibration as it has a dynamic imbalance. Static imbalance can be detected statically by determining which way up the object rolls over to rest. Dynamic imbalance can only be detected with the object rotating.
Methods for compensating for imbalance, including dynamic imbalance, are disclosed in U.S. Pat. No. 6,477,867 (Collecutt et al) and in U.S. Pat. No. 5,561,993 (Elgersma et al).
U.S. Pat. No. 6,477,867 discloses a balancing system where the output balance mass, in the form of water, is supplied to selected chambers at both ends of the drum to compensate for the calculated out of balance. The output of a force sensor at each end of the drum is processed to calculate an out of balance force as a rotating vector at each end of the drum.
Each end is treated separately. Two techniques are suggested to compensate for non-rigid systems, such as flexing of the machine cabinet or surroundings. An accelerometer may be provided adjacent each force sensor. The output of the accelerometer is included in processing the force sensor output to compensate for the force attributable to movement of the machine in the same measurement axis as the force sensor. Alternatively a method of calculating a system response is presented. The calculated system response is applied to the measured out of balance forces to calculate a balance correction.
While the systems presented in U.S. Pat. No. 6,477,867 are effective up to a certain degree there is a desire for further improvement in balancing accuracy so that the laundry machine drum may be accelerated to still higher speeds.
U.S. Pat. No. 5,561,993 discloses a balancing system where balance mass, in the form of water, is supplied to selected locations at both ends of the drum. The location and magnitude of the mass is calculated using Newton Raphsen iteration from a front force sensor input (vector), a back force sensor input (vector) a front acceleration sensor input (vector) and a back acceleration sensor input (vector). This iterative method involves applying known test masses at known locations. The system response to the test masses informs the calculation of a proposed counterbalance mass expected to reduce the sensor inputs.
The inventors believe that for the increased rotational speeds that are now desired the system response changes rapidly and unpredictably, so that the methods that require application of test masses are largely ineffective once the machine reaches these higher speeds.
In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents is not to be construed as an admission that such documents, or such sources of information, in any jurisdiction, are prior art, or form part of the common general knowledge in the art.